In recent years, an optical material called photonic crystals having a periodic refractive index distribution and techniques using the material have been receiving attention. The techniques include, for example, techniques for fabricating photonic crystals with an optical material, and techniques utilizing behavior of light in photonic crystals and techniques utilizing a phenomenon in which a luminous state of a luminous material existing in photonic crystals is controlled (E. Yablonovitch: “Phys. Rev. Lett” Vol. 58, p. 2059, 1987). A possibility of applying these techniques to the optical element is controversial.
In association with the technique of the optical element, so called DFB (distributed feedback) lasers effectively using one-dimensional periodic structures for semiconductor lasers and the like have gone into actual use. Basic studies for applying two-dimensional photonic crystals including a two-dimensional periodic arrangement of cylindrical pores to optical communication parts are vigorously conducted.
In two-dimensional photonic crystals, however, performance of light control in one non-periodic direction (direction of thickness in general) is inferior to performance of light control in other two periodic directions. This raises a problem when various optical elements, including optical communication parts, and systems using light are built. Some attempts are made to use three-dimensional (3D) photonic crystals forming periodic structures in all three directions.
Examples of three-dimensional photonic crystals, which have been developed to date, include crystals called lattice type crystals or wood pile type crystals (U.S. Pat. No. 5,335,240 (Ho et al.), Noda: “Photonic Crystals—Application, Technology and Physics,” p. 128, 2002, CMC press.), crystals fabricated by a micromechanic fabrication method (Hirayama et al: “Photonic Crystals—Application, Technology and Physics—,” p. 157, 2002, CMC press.), and crystals fabricated by a thin film deposition method called self cloning (S. Kawakami “Fabrication of submicrometre 3D periodic structures composed of Si/SiO2,” Electron. Lett., Vol. 33, pp. 1260-1261 (1997), International Patent Laid-open WO98/44368 publication and Sato: “Photonic Crystals—Application, Technology and Physics,” p. 229, 2002, CMC press.).
The biggest challenge in attempts to fabricate such three-dimensional photonic crystals is fabrication of a complicated three-dimensional structure in a fine period. The challenge is a technique for fabricating a three-dimensional form in which the period required in photonic crystals intended for near-infrared light wavelengths, visible light wavelengths, ultraviolet light wavelengths and the like, which are important especially in terms of application, is 1 μm or less, particularly on the order of 100 nm. Quality, such as dimensional accuracy and interface roughness, at 1 to 2 orders below the period is considered important. Values required as dimensional accuracy and roughness of the surface and the side face are, for example, about 1 to 10 nm. The roughness of the surface and the side face at this level causes scattering of light. Scattering of light causes a considerable loss in photonic crystals using multiple reflection and multiple beam interference as an operational principle, resulting in significant degradation in performance of the element.
For achieving a practical product, it is important that a plurality of elements can be fabricated at a time from a wafer material having a large area as in the case of many semiconductor parts. For example, if elements that are generally 10×10 μm2 to 1×1 mm2 in size can be fabricated from a wafer having an area of about 100×100 mm2, the number of elements that can be made from one wafer is increased and the cost is reduced.
It is very important that one element itself has a large area. This is because an element having a large area can provide a display or system in a form of one element.
For such needs, it is difficult to fabricate three-dimensional photonic crystals having a large area in a sufficiently high quantity and good quality using a conventional method. Specifically, in a conventional method in which the lattice of a compound semiconductor, such as GaAs, is welded, it is difficult to increase an area because the size of a substrate, such as GaAs, is limited, and it is difficult to reduce the cost of fabricating three-dimensional photonic crystals required to have a plurality of layers because such a substrate is expensive. Even if a method of depositing layers by micromechanic handling is used, the handling of a thin film having a large area is difficult in itself, and it is very difficult to maintain alignment across the large area.
If light having a wavelength of 1.3 to 1.5 μm, like near-infrared light for optical communications, is controlled, the thickness of each layer in a direction of deposition may be 0.3 to 0.5 μm. Thus, the existing method described above can be useful. However, if the element is considered for use with visible light, the thickness of each layer for about 0.4 μm equivalent to a wavelength of blue light must be 100 nm or less. Therefore, control and fabrication by the conventional method is difficult. In 3D photonic crystals, the thickness of each layer sensitively influences the optical performance even for near-infrared light in 3D photonic crystals. Thus, it is important to reduce the thickness of layers and provide high accuracy either in terms of improving accuracy or purposely adjusting the thickness of each layer finely to obtain high optical performance. In the conventional technique, it is difficult to provide a thickness on the order of 1 to 10 nm across a large area of 100×100 mm2.
In applications in which photonic crystals are used for routing elements for three-dimensional optical wiring and optical communications, not only a material should be arranged periodically, but also non-periodic structural parts called defects are introduced at desired positions to improve functions. In the conventional method, however, introduction of defects is difficult in itself, and it is difficult to perform position control of such defects across a large area.
For example, the self-cloning method has a problem in that it is difficult to introduce defects freely. Thus, functions cannot be improved.
As described above, the prior art described above cannot fully meet the strict requirements for 3D photonic crystals and devices and systems using the 3D photonic crystals.